Coordination control of a semicarbazide Schiff base ligand for spontaneous aggregation of a Ni2Ln2 cubane family: Influence of ligand arms and carboxylate bridges on the organization of the magnetic core

Article abstract of DOI:10.1039/c9nj05686f

A family of tetranuclear Ni-4f coordination aggregates (CAs) from the support of a carbazido ligand is described. The option of altering the 4f ions during synthesis within the family, without altering the core topology, gave the four members of the famil

Full text of DOI:10.1039/c9nj05686f

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DOI: 10.1039/C9NJ05686F
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Coordination Control of a Semicarbazide Schiff Base Ligand for
Spontaneous Aggregation of a Ni2Ln2 Cubane Family: Influence of
Ligand Arms and Carboxylate Bridges on the Organization of the
Magnetic Core
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mousumi Biswasa, E. Carolina Sañudob,c Jordi Cirerab,d and Debashis Ray*a
A family of tetranuclear Ni-4f coordination aggregates (CAs) from the support of a carbazido ligand is described. The
option of altering the 4f ions during synthesis within the family, without altering the core topology gave the four members
of the family. Tetranuclear (2:2) heterometallic compounds [NiII2LnIII2(μ3-L)2(μ3-OH)2(μ-OAc)3(AcO)(H2O)3]Cl2∙4H2O (Ln = Gd,
1; Tb, 2; Dy, 3; Ho, 4) have been assembled through the utilization of all the coordination sites of the anionic form of 1-(2-
hydroxy-3-methoxybenzylidene)semicarbazide (HL) and bridging supports from HO‒ and AcO‒ ions. The four metal ion
centers of two different types, are held together by two μ‒L‒, two μ3-HO‒ and three μ1,3-AcO‒ bridges to self-assemble into
Ni2Ln2O4 hetero-cubane structures. In each cube, LnIII ions are linked by two hydroxido and one phenoxido bridges and
each NiII ion is linked by two phenoxido and one hydroxido bridges, so that each hydroxido group in μ3-mode is linked to
two LnIII and one NiII and each phenoxido ion in μ3-mode linked two NiII and one LnIII. Variable temperature magnetic
properties have been examined to find the role of magnetic anisotropy for the single molecule magnet (SMM) behavior.
All of them show large ground state magnetic moment values and absence of out-of-phase ac susceptibility signals. The
absence
of
SMM
behavior
is
ascribed
to
the
lack
of
axial
magnetic
anisotropy.
(SMMs) are known to be compounds in which each molecular
entity is magnetically independent and able to slow magnetic
relaxation in the low temperature region.10-14 The long
relaxation times are obtained from a large anisotropy barrier,
but the process, like fast quantum tunneling of magnetization
(QTM), can often cancel the anisotropy barrier. A good SMM is
supposed to preserve their magnetic state for a long time even
after removal of external fields, which is only possible by
suppression of ground state quantum tunneling of
magnetization (QTM). Presence of unquenched orbital
momentum and anisotropic crystal field around the 4f ions
lead to the formation of SMMs from the synthesis of
coordination aggregates through utilization of spontaneous
self-assembly processes. Manipulation of reaction condition
and use of different combination of 3d and 4f ions lead to
particular course of aggregation between initially formed
fragments to provide new types of 3d-4f aggregates. Since the
easy-axis magnetic anisotropy, essential for SMM behavior,
can easily be produced for some lanthanide ions having large
ground state magnetic moment values.15,16 New structures of
these aggregates have shown attention for their potential in
high-density information storage device, quantum computer,
spintronics and magnetocaloric materials.17-21 3d ion based
molecular nanomagnets first identified in 1990s, specifically
SMMs and single-chain magnets (SCMs), display magnet-type
behavior without long-range magnetic ordering.22-24 Axially
symmetric crystal-field environments around the incorporated
Introduction
Coordination compounds of trivalent 4f ions are known to
have high coordination numbers (≥ 7) and flexible coordination
geometries, due to the higher positive charge, larger ionic radii
and hard Lewis acid character. These features are not
matching to the bivalent 3d series ions. Thus, bringing two of
these types of metal ions in close proximity through the
involvement and coordination control of chosen ligand anion
and standardization of exact reaction condition for the
accomplishment of the 3d-4f coordination aggregates (CAs) is
a task to the synthetic chemists. Such CAs, assembled by
organic phenol group bearing ligands incorporating
appropriate number of 3d and 4f ions, represent example of
multinuclear compounds having two or more types of metal
ions in single molecular entity.1-9 Single molecule magnets
a. Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302,
India. Email: dray@chem.iitkgp.ac.in
b. Departament de Química Inorgànica i Orgànica, Universitat de Barcelona,
Diagonal 645, 08028 Barcelona SPAIN.
c. Institut de Nanociència i Nanotecnologia, Universitat de Barcelona, (IN2UB)
08028 Barcelona SPAIN.
d. Institut de Química Teòrica i Computacional, Universitat de Barcelona (IQTC-UB),
Martí i Franqués 1-11, 08028, Barcelona, Spain
† Electronic supplementary information (ESI) available: Selected bond lengths and
angles Table S1, Shape analysis Table S2, additional structural figures Fig. S1–S6.
CCDC reference numbers 1941824-1941827. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/x0xx00000x
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3d and 4f ions, and strong exchange coupling between them
leads to the SMM behavior. The splitting of the degenerate
2J+1 ground state of 4f ions is highly sensitive to the
coordination environment and the donor atoms bound to the
metal ions, and any changes in those would be responsible to a
variation in the magnetic exchange interactions. Many
heterometallic 3d-4f complexes have been studied in detail
and reported in recent years from the use of new types of
ligand systems and utilization of room temperature solution
chemistry.25-27 In polynuclear NiII-LnIII complexes the
ferromagnetic exchange interactions between NiII and LnIII
center is responsible for generating functionally interesting
magnetic molecules.28-29 In particular the single-ion anisotropy
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of
a metal ion depends primarily on its coordination
environment and ligand field, while the molecular anisotropy
depends on ligand field, relative orientation of the individual
single-ion easy axes, magnetic coupling, and the overall
structural topology of the magnetic core. Thus synthetic
coordination chemists have the strong desire to design and
obtain new structural motifs to manipulate these
contributions.
We were interested to examine the heterometallic (3d-4f)
influence on the aggregation process, overall structure and
variable temperature magnetic behavior of the synthesized
compounds. The study recommended that discrete and easy
to synthesize heterometallic 2:2 cubanes serve as magnetically
active and useful materials whose structures are not guided
and apprehended by the ligand system and synthetic
protocols. Such ligand system has appealed significant
curiosity in recent past for the generation of new family of
structural as well as functional heterometallic aggregates.30
In this work, we have chosen 1-(2-hydroxy-3-
methoxybenzylidene)semicarbazide (HL) as a ditopic ligand
delivering μ3-phenoxido bridging mode for a family of Ni2Ln2
cubane-type complexes. Chosen reaction condition was
appropriate to obtain solvent water derived μ3-hydroxido
bridges and trapping of externally added carboxylate anions
for incorporation into the aggregates. Both anisotropic and
isotropic 4f ions, DyIII and GdIII were chosen to combine with
the NiII to observe their coordination compatibility in adjacent
pockets and the nature of magnetic exchange. Incorporation of
non‐Kramer 4f ion, HoIII having ground‐state multiplet of 5I8
has also been examined. In recent time HL was used by Chen
et al. to obtain [LnIII2] (Ln = Dy and Er)31, zigzag-shaped shaped
tetranuclear [Ni2Dy2] and [Co2Dy2],32 and trinuclear [Zn2Dy]33
complexes. None of these explorations could show the unusual
μ3-phenoxido bridge from L− and development of cubic
topology which is being reported in this work.
Scheme 1 Available binding sites of HL and observed binding
modes of L‒, AcO‒ and HO‒ groups
Experimental Section
Materials. All the starting materials and solvents for the
syntheses were analytically pure reagent grade and were used
as obtained from commercial houses without further
purification. The sources are: sodium acetate from SD Fine
Chem Pvt. Ltd., India, semicarbazide, lanthanide chlorides from
Alfa Aesar, India, triethylamine from SRL, India, nickel chloride
from E. Merck, India, methanol from Finar Ltd., India. All the
reactions and physical characterizations were carried out in
aerobic and normal laboratory conditions.
Synthesis of HL. The Schiff base ligand was obtained from a 1:1
condensation reaction of semicarbazide and o-vanillin
following a reported procedure.31,34 Yield: 78%. Obtained HL
was characterized by FT-IR and NMR spectral measurements
and used directly for reactions with metal ion salts without
further purification. Selected FT-IR peaks: (KBr disc, cm-1, vs =
very strong, br = broad, s = strong, m = medium, w = weak):
3436 (m), 3162(m), 1664(vs), 1600(s). 1H NMR (400 MHz,
DMSO-D6, ppm): δ 8.17 (1H, imine-H), δ 6.42 (2H, ─NH2), δ
2.49 (1H, ─NH), δ 10.27 (1H, phenolic OH), δ 6.42−7.37 (3H,
aromatic protons), δ 3.78 (3H, OCH3).
Four heterometallic [Ni2Ln2L2(μ3-OH)2(O2CCH3)4(H2O)3]Cl2·4H2O
[Ln= Gd (1), Tb (2), Dy (3), Ho (4)] complexes of cubic topology
have been synthesized and characterized. Detailed of the
syntheses, their structures, course of aggregation and
magnetic behaviors have been examined systematically.
Synthesis of complexes 1-4. All four complexes were obtained
by following a general procedure. To a colorless solution of HL
(0.1mmol) in MeOH (10 mL), a MeOH solution (2 mL) of
LnCl3∙6H2O (0.1mmol) was added under stirring condition for
an initial reaction in 1:1 molar ratio and the solution color
changed to light yellow. After 10 min of stirring neat Et3N
2 | J. Name., 2012, 00, 1-3
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(0.2mmol) solution was added drop by drop and the whole patterns were measured on Bruker AXS X-ray diffractrometer
reaction mixture was stirred for 1h. In the next step a MeOH using Cu Kα radiation (λ = 1.5418 Å) within the 5−50° (2θ)
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DOI: 10.1039/C9NJ05686F
4
solution (2 mL) of NiCl2∙6H2O (0.1mmol) was added to the range. Magnetic measurements were carried out in the Unitat
previous solution drop wise followed by the addition of de Mesures Magnètiques (Universitat de Barcelona) on
powder NaOAc (0.2mmol) and the entire reaction mixture was polycrystalline samples with a Quantum Design SQUID MPMS-
next stirred for 6h. The resulting clear green reaction mixture XL magnetometer equipped with a 5 T magnet. Diamagnetic
was then filtered and left undisturbed in a conical flask at corrections were calculated using Pascal's constants and an
room temperature for slow evaporation of solvent. From the experimental correction for the sample holder was applied.
reaction mixture green block shaped crystals, suitable for X-ray Thermogravimetric analysis of the complexes was carried out
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diffraction analysis, were obtained after 7 days.
using Netzsch STA 409 PC Luxx thermal analyzer within a
[Ni2Gd2(μ3-L)2(μ3-OH)2(μ-OAc)3(AcO)(H2O)3]Cl2·4H2O (1). HL temperature range of 30–700 °C and heating rate of 10 °C
(0.0209 g, 0.1mmol), GdCl3∙6H2O (0.0372 g, 0.1mmol), min−1.
NiCl2∙6H2O (0.0237 g, 0.1mmol), NEt3 (0.028 mL, 0.2mmol) and
NaOAc (0.0164 g, 0.2mmol). Yield: 0.0478 g, 73% (based on Single Crystal X-ray Diffraction Analysis. Suitable single
Gd). Anal. Calcd for C26H48Cl2Gd2N6Ni2O23 (1315.48 g mol-1): C, crystals of complexes 1-4 were selected for data collection on
23.74; H, 3. 68; N, 6.39. Found: C, 23.64; H, 3.28; N, 6.35. Bruker SMART APEX-II CCD X-ray diffractometer equipped with
Selected IR peaks (KBr cm-1; s = strong, vs = very strong, m = a graphite-monochromated Mo-Kα (λ = 0.710 73 Å) source
medium, br = broad ): 3370 (br), 1676(s), 1606 (m), 1560 (vs), with
1460(s), 1420 (vs), 1262 (m), 1226 (m), 1154 (m), 1082 (m), determination was performed by using XPREP and integration
1024 (m), 948 (w), 851 (w), 742 (m), 668 (w). of intensity for 25 reflections and reduction was performed
a counting time of 6s per frame. Space group
[Ni2Tb2(μ3-L)2(μ3-OH)2(μ-OAc)3(AcO)(H2O)3]Cl2·4H2O (2). HL using SAINT software. The structures were solved by direct
(0.0209 g, 0.1mmol), TbCl3∙6H2O (0.0373 g, 0.1mmol), methods SHELXS-201435, followed by refinement with full-
NiCl2∙6H2O (0.0237 g, 0.1mmol), NEt3 (0.028 mL, 0.2mmol) and matrix least squares on F2 using SHELXL-(2014/7)36 software
NaOAc (0.0164 g, 0.2mmol). Yield: 0.0492 g, 75% (based on available with WINGX system Version 2014.137 and Olex-2
Tb). Anal. Calcd for C26H48Cl2Tb2N6Ni2O23 (1318.83 g mol-1): C, software38 to mask the disordered solvent molecules. Multi-
23.68; H, 3. 67; N, 6.37. Found: C, 24.01; H, 3.70; N, 6.59. scan absorption correction was performed using SADABS39
Selected IR peaks (KBr cm-1; s = strong, vs = very strong, m = program. All the non-hydrogen atoms were refined
medium, br = broad ): 3370 (br), 1674 (s), 1605 (m), 1551 (vs), anisotropically and the hydrogen atoms were placed in
1460 (s), 1412 (vs), 1260 (m), 1225 (m), 1151 (m), 1080 (m), calculated positions and refined with fixed geometry and
1022 (m), 948 (w), 847 (w), 737 (m), 663 (w).
riding thermal parameters on their carrier atoms. The
[Ni2Dy2(μ3-L)2(μ3-OH)2(μ-OAc)3(AcO)(H2O)3]Cl2·4H2O (3). HL locations of heaviest atoms (Ni, Ln) were determined easily
(0.0209 g, 0.1mmol), DyCl3∙6H2O (0.0377 g, 0.1mmol), and positions of O, N, C were determined from difference
NiCl2∙6H2O (0.0237 g, 0.1mmol), NEt3 (0.028 mL, 0.2mmol) and Fourier maps. The crystallographic figures were generated
NaOAc (0.0164 g, 0.2mmol). Yield: 0.0512 g, 77% (based on using Mercury, DIAMOND40 and POV-ray41 software.
Dy). Anal. Calcd for C26H48Cl2Dy2N6Ni2O23 (1325.98 g mol-1): C, Crystallographic data refinements details are summarized in
23.55; H, 3.65; N, 6.34. Found: C, 23.77; H, 3.44; N; 6.18. Table 1, and selected bond distances and angles are given in
Selected IR peaks (KBr cm-1; s = strong, vs = very strong, m = Table S1. CCDC 1941824-1941827 contain the supplementary
medium, br = broad ): 3366 (br), 1678(s), 1606 (m),1560 (vs), crystallographic data for complexes 1-4. The data can be
1461 (m), 1420 (vs), 1262 (m), 1226 (m), 1152 (m), 1082 (m), obtained free of charge from The Cambridge Crystallographic
1024 (m), 948 (w), 848 (w), 740 (m), 666 (w).
Data Centre via www.ccdc.cam.ac.uk/structures.”
[Ni2Ho2(μ3-L)2(μ3-OH)2(μ-OAc)3(AcO)(H2O)3]Cl2·4H2O (4). HL
(0.0209 g, 0.1mmol), HoCl3∙6H2O (0.0379 g, 0.1mmol), Continuous Shape Measures. The Continuous Shape Measures
NiCl2∙6H2O (0.0237 g, 0.1mmol), NEt3 (0.028mL, 0.2mmol) and (CShM) have been performed using the SHAPE 2.1 software.42-
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NaOAc (0.0164 g, 0.2mmol). Yield: 0.0500 g, 75% (based on
To retrieve the coordination sphere of the lanthanide ion, a
Ho). Anal. Calcd for C26H48Cl2Ho2N6Ni2O23 (1330.84 g mol-1): C, search for all bonded atoms was done using the criteria that
23.47; H, 3.64; N, 6.31. Found: C, 23.15; H, 3.51; N, 6.47. the atom-atom bond length has to be below the sum of the
Selected IR peaks (KBr cm-1; s = strong, vs = very strong, m = atoms covalent radii.45 This procedure recovers a total of eight
medium, br = broad ): 3368 (br), 1678 (s), 1606 (m), 1560 (vs), oxygen atoms around each lanthanide ion. As reference
1462 (m), 1414 (vs), 1262 (m), 1226 (m), 1152 (m), 1082 (m), shapes, a total of thirteen ideal polyhedral structures having
1024 (m), 948 (w), 848 (w), 740 (m), 668 (w).
eight vertices have been used.
Physical Measurements. Elemental analyses (C, H, N) for 1-4
were performed on a Perkin-Elmer (model 240C) elemental
analyzer. FT-IR spectra were recorded on a Perkin-Elmer RX1
spectrometer and solution electronic spectra were measured
using Shimadzu UV 3100 UV–Vis–NIR spectrophotometer. The
diffuse reflectance spectra (DRS) were measured using a CARY-
5000 UV-vis-NIR spectrophotometer. Powder X-ray diffraction
Results and discussion
Synthesis, Isolation and Crystallization from Reaction
Medium. Ligand HL used in this work has adjacent tridentate
(ONO) and bidentate (OO) coordination pockets appropriate
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did not provide the hitherto unknown dinuclVeiaewr Arctioclme Opnlleinxe
[NiLn(μ-L)(μ-OH)(OOCCH3)2(H2O)n]Cl or any above mentioned
ligand anion bound species.46
DOI: 10.1039/C9NJ05686F
Green block-shaped crystals of 1-4 in 73-78% yield were
isolated from the reaction mixture after different time
intervals of 7 to 10 days. X-ray structure analysis of the single
crystals confirm the final product as [Ni2Ln2(μ3-L)2(μ3-
OH)2(O2CCH3)4(H2O)3]Cl2∙nH2O. (Scheme 3) Elemental analysis
and physical characterizations of the products confirmed the
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Scheme 2 One pot synthesis of HL
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for the binding of 3d and 4f ions. HL was synthesized from formation of the tetranuclear cationic aggregates as chloride
Schiff base condensation reaction between o-vanillin and salts.
semicarbazide at room temperature in MeOH medium. Solid State Characterizations
(Scheme 2) The ligand is chosen to use the (ONO) pocket to FT-IR Spectra. For all the four complexes 1-4, a broad band of
bind a smaller and ‘border line’ NiII ion and the adjacent (OO) medium intensity within 3660-3370 cm−1 range is attributed to
site was made available to trap a bigger and ‘hard’ LnIII (Gd, Tb, the ṽO-H and ṽN-H stretches. In case of free HL, two
Dy and Ho) ions. Hard O-donors are known to favor the characteristic medium intensity peaks were found at 3434 and
coordination of LnIII ions whereas border-line N-donors have 3278 cm−l due to the ṽO-H and ṽN-H stretches. The characteristic
higher affinity to bind the 3d MII ions. Use of LnCl3 salts in 1:1 ṽC=N stretching frequency from the NiII bound L‒ is observed at
molar ratio with HL in the chosen reaction medium inhibit the 1605 cm−l, confirming a small shift from the corresponding
formation of any kind of [Ln2] type complex as obtained by value at 1598 cm−l for HL. The LnIII bound ṽC=O stretching
others.27 In situ generation of hydroxide ions in the presence frequencies on the other hand for the entire family of
of available acetate ions drove the reaction in solution complexes appear within 1674-1678 cm−l, compared to the
−
1 47
followed by crystallization to the Ni2Ln2 aggregate formation.
free ligand value of 1669 cm . For the whole family, the
and symmetric (ṽs(COO) stretching
The reactions of HL from sequential addition of LnCl3∙6H2O and asymmetric (ṽas(COO)
)
)
NiCl2∙6H2O in MeOH in the presence of NEt3 and NaOAc has vibrations of three bridging acetato groups are detected at
been explored for the generation of solid crystalline 1560, 1551, 1560 and 1560 cm−1, and 1420, 1412, 1420, 1414
coordination
OH)2(O2CCH3)4(H2O)3]Cl2∙nH2O (Ln = Gd,
All the four complexes ( ) were isolated as single-crystalline agreement with the presence of μ1,3-carboxylato bridges
aggregates
[Ni2Ln2(μ3-L)2(μ3- cm−1, respectively for
; Tb, ; Dy, ; Ho,
1
-
4
. The differences in respective cases
1
2
3
4
). (Δṽ = ṽas(COO) − ṽs(COO)) are within 139-146 cm−1, which is in
1-4
materials, and good quality single crystals were selected for X- between NiII and LnIII centers. The symmetric (ṽs(COO)) stretching
ray structure determinations. The chemical reaction involved vibrations for terminal AcO‒ groups in 1-4 appear at 1228,
during the generation of the complexes is summarized in eq 1. 1224, 1228 and 1228 cm−1. As a result the higher magnitude of
Δṽ values of 327-332 cm−1 confirm the terminal carboxylate
2HL + 2Ln퐶푙3 ∙ 6퐻2푂 + 2푁푖퐶푙2 ∙ 6퐻2푂 + 4푁퐸푡3 + 4퐶퐻3퐶푂푂푁푎
푀푒푂퐻
→
binding to the LnIII centers.48
[ ]
푁푖2퐿푛2(휇3 − 퐿)2(휇3 − 푂퐻)2(푂푂퐶퐶퐻3)4(퐻2푂)3 퐶푙2 ∙ 푛퐻2푂 +
Electronic Spectra. Solid state DRS for 1-4 were recorded in
1200-200 nm range. (Figure S2) The presence of distorted
octahedral NiII centers, bridged to the LnIII centers, showed
( ) ( )
4 푁퐸푡3퐻 퐶푙 + 4푁푎퐶푙 + 19 − 푛 퐻2푂 … … .. (1)
In solution L− anion initially binds LnIII and NiII to generate characteristic peaks for the characteristic ligand field
reactive {NiLn(μ-L)} fragments, having hydroxide and acetato transitions. Broad absorption bands with maxima at 987, 982,
3
links gave [NiLn(μ-L)(μ-OH)(OOCCH3)2(H2O)n]1+. Spontaneous 987 and 989 nm can be assigned to the spin allowed A2g (F) →
dimerization of two such units resulted 1-4 (Scheme S1).
Addition of bidentate ligands such as bpy, o-phen, acacH high energy bands with maxima at 617, 617, 613 and 624 nm,
3T1g (F) transitions of the complexes 1-4, respectively. The next
3
3
(acetylacetone) and oxine (8-hydroxy quinoline) in the reaction respectively can be assigned to the A2g (F) → T1g (P)
medium immediately after the complete addition of metal ions transitions. Unfortunately, the lowest energy 3A2g (F) → 3T2g (F)
transition was missing for all the complexes at the used
thickness of solid state sample materials. Very weak transitions
at 766, 767, 759 and 766 nm for
1
-
4
were appeared as a
3
1
shoulder for the spin forbidden A2g (F) → Eg (D) transitions.
Highly intense PhO− → NiII LMCT transitions were observed at
369, 365, 365 and 369 nm for 1-4, respectively. The intra-
ligand π → π* transitions centered on C=C, C=O and C=N
functions within the NiII-bound ligand anion back bone were
found at 277, 281, 275 and 274 nm, respectively for 1-4
.
Powder X-ray Diffraction and thermogravimetric analysis. The
identical nature and composition of the as synthesized solid
state polycrystalline samples and those obtained from single-
crystal X-ray structure determination within the same family of
Scheme 3 Synthesis of compounds 1-4
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compounds have been compared from the powder X-ray diffraction peak is exclusively determined by the size and
View Article Online
diffraction (PXRD) analysis and TGA analysis. The PXRD shape of the unit cell of theDOcI:r1y0s.t1a03ll9in/Ce9NJp0h56a8s6eF.
patterns have been analyzed to identify and characterize the Thermogravimetric analysis (TGA) showed
a
similar
individual phases of solid crystalline compounds of 1-4. The decomposition pattern for the complexes 1-4 towards heating
patterns for 1-4 are shown in Figure S3 in SI. These patterns of the samples for weight loss. A weight loss in 40–125 °C
are matching nicely with the simulated ones obtained from the range was observed uniformly for all the complexes due to the
single-crystal X-ray diffraction data. The obtained 2θ values removal of solvents of crystallization. The next higher
corresponding to the lattice spacing and the relative intensity temperature loss is attributed to the removal of water
of the diffraction lines found are revealing the particular phase molecules coordinated to the lanthanide ions. (Figure S4 in SI)
of the compounds 1-4. The position of a characteristic
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Table 1 Crystal data and structure refinement summary of complexes 1-4
Complex
Empirical formulaa
Formula weighta
Crystal system
Space group
a (Å)
1 (Ln=Gd)
C26H42Cl2Gd2N6Ni2O20
1261.46
2 (Ln=Tb)
C26H42Cl2Tb2N6Ni2O20
1264.81
3 (Ln=Dy)
C26H42Cl2Dy2N6Ni2O20
1271.96
Monoclinic
P21/n
4 (Ln=Ho)
C26H42Cl2Ho2N6Ni2O20
1276.83
Monoclinic
P21/n
Monoclinic
P21/n
Monoclinic
P21/n
13.8573(8)
23.9482(14)
14.3941(8)
105.322(2)
4607.0(5)
4
13.757(5)
24.089(14)
14.377(5)
104.228(10)
4618(3)
13.419(17)
23.74(3)
14.18(3)
103.61(3)
4392(12)
4
13.421(3)
23.686(5)
14.348(3)
100.79(3)
4480.4(17)
4
b (Å)
c (Å)
β (o)
Volume (Å3)
Z
4
Dcalcd (g cm-3)
Absorption coefficient (mm-1)
F (000)
1.816
1.819
1.921
1.893
3.837
4.018
4.407
4.516
2464
2480
2480
2496
Temperature/K
293(2)
298(2)
293(2)
296(2)
Reflections collected/unique
Parameters
55213 / 9023
533
59727 / 10195
528
44294 / 9236
526
38048 / 9920
537
Goodness-of-fit (F2)
1.026
1.022
1.122
1.063
Rint
R1; wR2 [I >2σ(I)]
CCDC No.
0.0713
0.0687, 0.2064
1941824
0.0658
0.0607, 0.1787
1941825
0.0802
0.0779, 0.2168
1941826
0.0646
0.0391, 0.0949
1941827
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Descriptions for the Crystal Structures. Single crystals suitable
for X-ray analysis were directly obtained from the mother
liquor after 7 to 10 days in the four cases through evaporation
of the solvent mixtures.
View Article Online
DOI: 10.1039/C9NJ05686F
[NiII TbIII2(μ3-L)2(μ3-OH)2(μ-OAc)3(AcO)(H2O)3]Cl2·4H2O (2). In
2
this section detail discussion on the X-ray structure is made for
2
as a representative case. All the tetranuclear [Ni2Ln2]
Fig. 2 Highlighted cube core of
2
(middle) and the bridged
coordination environments around two NiII (left) and two TbIII
(right) centers. Color code as above
compounds (1-4) crystallize in monoclinic P21/n space group
with Z = 4. Crystal data and the important refinement
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parameters are presented in Table 1. The perspective view of
2
is shown in Figure 1a. Selected bond distances and angles are
presented in Table S1. All the CShM values for the Ln ions are
to one imine N (Ni1
−
N1, 2.017(7) Å), two bridging phenoxido O
O1, 2.059(6) Å; Ni1 O2, 2.230(5) Å), one hydroxido O
O3, 2.012(6) Å), one acetato O (Ni1 O10, 2.050(6) Å) and
O15, 2.072(7) Å). The hydroxido
donor showed shortest Ni O distance and one of the
phenoxido donors the longest.
The eight-coordinate (O8) environment around Tb1 (distorted
triangular dodecahedron, S(TDD-8)= 0.65) and Tb2 (distorted
square antiprism, S(SAPR-8)= 0.76 ) are accomplished by one
bidentate OO donor part from L−, two hydroxido bridges, two
acetato bridges (Tb···Tb and Tb···Ni) and two terminal
monodentate water to Tb2 center and one terminal
monodentate water and an acetate anion to Tb1 center. The
CShM analysis revealed that the structures cannot be
described as intermediate one following any interconversion
(Ni1
(Ni1
−
−
−
provided in Table S2. The analysis of the X-ray structure for
2
revealed that each L− unit, delivering NO3 donor set, binds one
TbIII and one NiII center in two adjacent pockets around the
bridging phenoxido group giving a {NiIITbIIIL} fragment. To the
NiII center the ONO pocket of L− showed meridional
coordination. The adjacent bidentate OO pocket on the other
hand is used to bind the bigger and oxophilic TbIII ion. In situ
generation of HO− ion from the solvent medium then holds the
previous entity as {NiIITbIIIL(OH)}. Water molecules from the
reaction medium and available AcO− ions occupy the vacant
coordination sites around NiII and TbIII to provide
{NiIITbIIIL(OH)(AcO)m(H2O)n}. Self-aggregation of two such units
−
one semicarbazide O (Ni1
−
−
provide the cubane core of 2 (Figure 1b).
The tetranuclear [Ni2Tb2] core thus assembled around two μ3-
PhO− (O1 and O2) parts of L− and two water derived μ3-HO−
(O3 and O4) groups at the four alternate corners of the cube.
The aggregation is supported by the μ-O,O′-acetato bridges in
three (one Tb···Tb and two Ni···Tb) faces of the cube. Higher
(eight) coordination number around the TbIII centers are
fulfilled by the monodentate coordination from H2O and AcO−.
Within the cube structure, the Ni−O distances vary within
2.011(6)-2.230(5) Å, due to presence of three types of O donor
path. In 2 the phenoxido O donors (O1 and O2) provide shorter
distances (Tb1–O2, 2.474(5) Å; Tb2–O1, 2.486(6) Å), in
comparison to the separations from –OMe (O5 and O6) groups
(Tb1–O5, 2.556(7) Å; Tb2–O6, 2.620(9) Å). The Tb–O distances
for hydroxido bridges are in 2.341(6)-2.411(7) Å range. Unlike
the known 3d ion based cube structures, the structure of
2 has
the shortest Ni···Ni separation of 3.225 Å. Presence of bigger
TbIII ions increases the Tb···Tb separation to 3.779 Å. On the
four other faces, the Ni···Tb separations are slightly longer
than the Ni···Ni separation (3.225 Å) and ranging from 3.396 to
3.598 Å. The three acetato-bridged faces show shorter
separation (Figure 4). The μ3-PhO− (O1 and O2) part of L−
toward NiII ions record two shorter in plane and two longer out
of plane Ni−O bonds in 2.050(6), 2.059(6) Å and 2.227(6),
2.230(5) Å range. Two μ3-HO− bridges provide shorter Ni–O
bonds at 2.011(6), 2.012(6) Å and longer Tb–O bonds at
2.341(6)-2.411(7) Å. The presence of three AcO− bridges on
three faces of the cube register Ni−O and Tb−O bonds in
2.050(6)-2.059(6) Å and 2.320(8)-2.367(7) Å range. The μ3-
PhO− and μ3-HO− bridges register different magnitudes of
atoms and Ni−N bond lengths remain within 2.017(7)-2.024(7)
Å, which are in normal range for distorted octahedral
coordination environment around any NiII center (Figure 2,
left). Each NiII center in NO5 coordination environment bound
Ni−O−Ni 97.50(2)
99.30(2) and Tb−O−Tb 104.00(2)
For the distorted square anti-prism coordination geometry
around Tb2, is considered as the angle between the S8 axis
of the square anti-prism and the central atom-ligand bond
(Figure S6).49,50 The same can be considered through
= 2
where is the angle between the opposite bonds within one
hemisphere. For Tb2, the magnitude of varies from 50.4 to
64.28 , with the smallest one as (O4, O6) and largest one as
(O3, O18)

and 97.70(2)

,
Ni−OPh−Tb 96.20(2)
-


and 106.80(2)

angles.

(a)
Fig. 1 (a) Perspective view of 2. Hydrogen atoms and solvent
molecules are omitted for clarity; (b) core view of with
(b)

,
2

partial atom numbering scheme. Color scheme: cyan, Tb;
green, Ni; red, O; blue, N; gray, C


.
6 | J. Name., 2012, 00, 1-3
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As the temperature decreases, the
and
indicating the onset of oDvOerI:a1l0l .10f3e9r/rCo9mNaJ0g5n6e8t6icF
interactions between the NiII and LnIII centers. For
and the
shape of the T product curve is typical of the lanthanide ions
T product inVcireewaAsrteicslefOonrlin1e
2
,
3
4

6
having strong spin orbit coupling and it shows the thermal
depopulation of the Mj states. This is consistent with weak
ferromagnetic coupling, however, the possibility that one of
the exchange pathways is antiferromagnetic cannot be ruled
out. The three exchange pathways J1(Ni‒Ni), J2(Ln‒Ln) and
J3(Ln‒Ni) were considered within the [Ni2Ln2O4] core, as shown
7
8
9
10
11
12
13
14
15
16
17
18
19
(a)
in Scheme 5. For the series
1-4 the Ln‒OH‒Ln angles are
between 104‒108 and the Ln‒OH‒Ln‒OH diamond is not flat,
with an average Ln‒OH‒OH‒Ln torsion angle of 153. The Ln‒
O‒Ni angles are on average 98, while the Ln‒OH‒Ni angles are
between 101 and 110. With these structural parameters in
hand, a weak ferromagnetic coupling should have been
expected for adjacent Ln∙∙∙Ni centers. The syn, syn-AcO−
bridges between Ln∙∙∙Ni and Ln∙∙∙Ln parts are known to
mediate antiferromagnetic coupling. Thus for Ln∙∙∙Ln and
Ni∙∙∙Ln both the feasible pathways, ferromagnetic via the OH-
or O bridge, and antiferromagnetic via the syn, syn-AcO−
bridge will finally result in only very weak exchange coupling
(b)
Fig. 3 (a) Distorted octahedral coordination environment of
adjacent NiII centers; (b) distorted triangular dodecahedron
and distorted square antiprism coordination geometry around
Tb1 and Tb2, respectively
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values. The Ni−O−
Ni angles, in the 97 to 98 range, and the
In case of Tb1 the two square planes of any square antiprism
like structure deviate from planarity to provide two triangular
parts instead and resulting in a distorted trigonal dodecahedral
structure. The situation is clueless with regard to the binding
of L− and ancillary bridges for different amounts of distortion
around the two TbIII centers. The different magnitude of
distortion around the NiII centers is less pronounced.
Along the series, the shortest Ni1∙∙∙Ni2 and longest Ln1∙∙∙Ln2
separations are in the ranges of 3.181-3.255 Å and 3.737-3.786
Å, respectively. For the entire family of Ni2Ln2 complexes the
variety of NiII∙∙∙LnIII separations fall in 3.355-3.598 Å range
along the 4f series.
Ni···Ni distance of 3.2 Å are in good agreement for reported
values of Ni4O4 cube structures. In the 97-99 range the
coupling can be either ferromagnetic or antiferromagnetic for
this angular range, and in all cases the values are close to
zero.51,52 For the GdIII complex
1, the magnetic data has been
fitted using the software PHI.53 The program uses the
Hamiltonian of Eq. 2 and 3.
Eq .… (2)
Magnetic Properties
Susceptibility data for
field: 0.5 T for and 0.3 T for
data set were collected with applied field values of 295 Oe (
) and 275 Oe ( ). As can be seen in Figure 5, both the data
sets overlap. The T products at 300 K have values as shown in
1
-
4
were collected at 2 to 300 K (applied
Eq ....(3)
1
,
3
2, 4
). Below 30 K an additional
To avoid overparametrization, the susceptibility data for
complex 1 was fitted using only the exchange Hamiltonian with
1
,
3
2, 4
fixed g values for NiII and GdIII at the default software values.
The three fitting parameters used were J1(Ni‒Ni) = ‒0.91 cm-1,
J2(Gd‒Gd) = 0.14 cm-1 and J3(Ni‒Gd) = 0.02 cm-1, as shown in
Scheme 5. According to the fitting, the weakest exchange is
that between Ni-Gd. The exchange values obtained are in
agreement with the expected values discussed previously.
Bear in mind that even the two Gd ions are not
crystallographically equivalent, they are considered equivalent
in this simplified model. The fitting is shown in Figure 5 as a
solid line. The same values were used to calculate the

Table 2 and are in agreement with the expected values for two
NiII centers in octahedral environment with S = 1 and g = 2.0
and two LnIII ions.
magnetization as a function of field for the complex 1, and the
calculated magnetization is shown in Figure 6 as a solid line.
There is good agreement between calculated and
experimental data. The coupling model results in a spin ground
state for
the very small values of the exchange constants. Similar
situations can be expected for the complexes
1 of ST = 7 that is not well isolated in energy due to
Fig. 4 Three acetato bridged structure of
2 with different M‒
1-4.
O‒M angles within the cube
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coordination environment, as shown in FigurVeiew3Arti(cble)Onflioner
DOI: 10.1039/C9NJ05686F
complex
apply and the anisotropy of complex
The large spin of Gd has been shown to be the key to good
SMM properties if combined with highly anisotropic
2. Since Gd is isotropic, such considerations do not
1
arises from the Ni2 unit.
a
transition metal unit, as in a Co-Gd complex reported by
Sañudo et al, the analogous Ni complex lacked sufficient
anisotropy and was not an SMM.55 Since in the complexes
reported here the Ni-Ni coupling is antiferromagnetic, complex
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Scheme 5 Coupling interactions within the magnetic [Ni2Ln2O4]
core. The three exchange pathways J1(Ni‒Ni) (red), J2(Ln‒Ln)
(blue) and J3 (Ln‒Ni) (green)
1
is not an SMM. The software MAGELLAN56 can be used to
compute the anisotropy axes on DyIII ions. Thus we used
MAGELLAN for complex in order to confirm the lack of
3
Heterometallic 3d-4f SMMs are relatively common, in
particular when the ground state magnetic moment is large as
for the Ni—Gd complex reported here.25 The other complexes
in the series should also display ground states with large
magnetic moments given the magnetization vs. field behaviour
at 2 K. Thus, the relaxation dynamics for the complexes 1-4
anisotropy of the Ni2Ln2 core. The software uses a purely
electrostatic model and the results are shown in Figure 7. The
solid red lines indicate the anisotropy axes on the two DyIII ions
in the complex. Clearly these axes are not aligned. This fact
combined with the two different ligand environments for Ln1
and Ln2 result in a lack of SMM properties for these
complexes.
were studied in
a commercial SQUID. Out-of-phase ac
susceptibility signals were not observed for any of the
complexes. Since the complexes reported here have significant
(if not well isolated) spin ground states, the lack of SMM
behavior must be attributed to the lack of axial magnetic
anisotropy in these species. Following the crystal structure
description, it is clear that only one LnIII ion in the complex
shows the appropriate ligand field to present SMM properties.
For Tb, Dy and Ho, Rinehart and Long predicted strong
anisotropy with an axial distribution of ligands due to the
oblate distribution of electron density.54 According to the
crystal structure of these complexes, only one of the
The Ni2Ln2 cube complexes synthesized by Liu et al. showed Ni∙∙∙Ni
antiferromagnetic interactions with J(Ni‒Ni) value of ‒3.91 cm-

1
within ˂Ni-O-Ni angle of 98.44 and 99.72 .52 Other reported
Ni2Ln2 cube structures showed ferromagnetic interactions
between Ni∙∙∙Ni with J(Ni‒Ni) values of +3.28, +3.30 and +5.20
cm-1 within comparable ˂Ni-O-Ni angle of 99 and ˂Gd-O-Gd
angle of 109 and weak Ni∙∙∙Gd ferromagnetic interactions,
where none of the complexes showed SMM behavior.57-59
Hence, from these reports of Ni2Ln2 cubanes it is clearly seen
that only weak exchange couplings are operative which clearly
suggest a strong dependence on the geometrical parameters
distorting the cubane structure Table S3.
lanthanide(III) ions presents such
a strong axial ligand
arrangement in a square antiprism coordination sphere while
the other lanthanide in the complex is in a less axial
Table 2 Comparison of calculated and experimental T data
Calculated T at 300 K (cm3 K mol-1)
Experimental T at 300 K (cm3 K mol-1)
Complex
Ni(II) (S = 1, g = 2.0)
1
2
3
17.8
25.26
30.32
17.41
25.31
29.38
Gd(III) ion (8S7/2, S = 7/2, L = 0, g = 2.0)
Tb(III) ion (7F6, S = 3, L = 3, J = 6 and gJ = 3/2)
Dy(III) ion (6H15/2, S = 5/2, L = 5, J = 15/2 and
gJ = 4/3)
4
30.0
29.17
Ho(III) ion (5I8, S = 2, L = 6, J = 8 and gJ = 10/8)
8 | J. Name., 2012, 00, 1-3
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ligand system. Standardization of the reaction condition
DOI: 10.1039/C9NJ05686F
provided the option for systematic replacement of the
View Article Online
lanthanide ions within the family, without altering the
paramagnetic core structure. We were successful in exploiting
all the coordination sites of the used ligand system while
trapping two 3d and two 4f ions. The magnetic properties of 1-
5
6
7
8
9
4
indicated a combination of weak exchange couplings, and
10
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13
14
15
16
comparison with other Ni2Ln2 cubanes known in the literature,
suggest a strong dependence of the coupling behavior on the
structural distortions. Out-of-phase ac susceptibility signals
were not observed for any of the complexes in the family, not
even for the Dy complex 3. Out-of-phase ac susceptibility
signals were also not observed for any of the complexes in the
family. For 3
, the anisotropy axes on the two DyIII ions are not
Fig. 5 Plot of T products vs. T for the complexes 1-4. The solid
17
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aligned and the anisotropy of the complex is not large. The
entire family of the complexes lacked axial anisotropy and
SMM behavior even though the ground state magnetic
moment values are quite large.
line is the best fitting to the experimental data for complex
using PHI software
1
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
MB is thankful to the University Grants Commission, New
Delhi, India, for financial support. We are also thankful to the
DST, New Delhi, India, for providing the single-crystal X-ray
diffractometer facility at the Department of Chemistry, IIT
Kharagpur, under its FIST program. ECS acknowledges the
financial support from the Spanish Government (Grant
CTQ2015-68370-P).
Fig. 6 Magnetization vs. field plots for complexes 1-4. The solid
line is the calculated magnetization data for complex
PHI software
1 using
Notes and references
1
2
3
S. Osa, T. Kido, N. Matsumoto, N. Re, A. Pochaba and J.
Mrozinski, J. Am. Chem. Soc., 2004, 126, 420−421.
R. Sessoli, D. Gatteschi, A. Caneschi and M. A. Novak,
Nature., 1993, 365, 141−143.
J. -B. Peng, Q.-C. Zhang, X.-J. Kong, Y.-Z. Zheng, Y.-P. Ren, L.-
S. Long, R.-B. Huang, L.-S. Zheng and Z. J. Zheng, J. Am. Chem.
Soc., 2012, 134, 3314−3317.
4
5
J. Goura, R. Guillaume, E. Rivere and V. Chandrasekhar,
Inorg. Chem., 2014, 53, 7815 −7823.
K. C. Mondal, A. Sundt, Y. Lan, G. E. Kostakis, O. Waldmann,
L. Ungur, L. F. Chibotaru, C. E. Anson and A. K. Powell,
Angew. Chem., 2012, 124, 7668-7672.
Fig. 7 Illustration of the structure of
3 with the anisotropy axes
(shown in red) calculated using the MAGELLAN software.
Hydrogen atoms are omitted for clarity. DyIII shown as light
green (large spheres), NiII as dark green (smaller spheres)
6
7
8
9
T. Kajiwara, M. Nakano, K. Takahashi, S. Takaishi and M.
Yamashita, Chem. Eur. J., 2011, 17, 196 – 205.
L. Zhao, J. Wu, H. Ke and J. Tang, Inorg. Chem., 2014, 53
,
3519−3525.
J.-P. Costes, F. Dahan, L. Vendier, S. Shova, G. Lorusso and M.
Evangelisti, Dalton Trans., 2018, 47, 1106–1116.
R. Modak, Y. Sikdar, A. E. Thuijs, G. Christou and S. Goswami,
Inorg. Chem., 2016, 55, 10192−10202.
Conclusions
Using the semicarbazide tail bearing Schiff base ligand system
for the first time we were able to synthesize a family of Ni-4f
aggregates having cube topology. This is the first report of
such cubic aggregate from the use of semicarbazide containing
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DOI: 10.1039/C9NJ05686F
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A new family of tetranuclear [Ni2Ln2O4] cubane complexes has been assembled from the
coordinating support of a carbazido ligand. The opportunity of altering the 4f ions during
synthesis within the family, without changing the core topology provided the family to show
weak ferromagnetic and antiferromagnetic interactions.
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